1. Field of the Invention
The present invention relates to a surface illuminator utilizing an array of discrete light sources and a liquid crystal display having the same.
2. Description of the Related Art
A liquid crystal display is provided with a surface illuminator on a front side or back side of a liquid crystal display panel thereof. Backlight units which are surface illuminators disposed on the back side of a panel include side light (edge light) types including a light source disposed along a side edge of a light guide for guiding light and direct types including a light source disposed directly under a liquid crystal display panel. Referring to the use of those types of illuminators in general, side light type backlight units are used in liquid crystal displays having screen sizes of 20 (20 inches in the diagonal direction) or less and liquid crystal displays which must have an especially small thickness. While either type of illuminators generally employ a cold-cathode tube as a light source, it is not preferable to use a cold-cathode tube that utilizes mercury under the recent circumstance in which environmental problems are taken seriously. For this reason, various light sources such as mercuryless fluorescent tubes and LEDs (light-emitting diodes) have been developed as light sources to replace cold-cathode tubes, and LEDs are regarded as promising next generation light sources.
When LEDs are used as a light source of a side light type backlight unit, the light source may be configured by arranging a plurality of white LEDs or arranging a plurality of sets of LEDs, each set consisting of monochromatic LEDs emitting light in different colors (e.g., red (R), green (G), and blue (B)). A white LED is a combination of a yellow-emitting fluorescent body and a blue-emitting LED, and it has relatively small variation in the color of emission. Backlight units utilizing a combination of R, G, and B monochromatic LEDs are attracting keen attention for their capability of achieving a vast range of color reproducibility which is not achievable with white LEDs.
For example, in the case of a primary color emitting LED set utilizing R emission LEDs, G emission LEDs, and B emission LEDs in combination, since the width of each LED is about 10 mm, LEDs in the same color are disposed at intervals of 30 mm or more from each other. Therefore, a method of mixing emission colors is needed. Lumileds Lighting LLC has proposed a system (sub light guide system) in which a light guide region for mixing emission colors is not used as a display area (see Non-Patent Document 1). Non-Patent Document 1 discloses a backlight having a double light guide structure in which a sub light guide is used. The sub light guide mixes RGB colors and causes resultant light in uniform white color to impinge on a main light guide located above the same. There is a problem in that the system as a whole results in very low utilization of light because light enters the sub light guide from LEDs and enters the sub light guide from the main light guide at low efficiency. Since the low utilization of light necessitates an increase in the power supplied and consequently necessitates countermeasures against heat, a problem also arises in that the size of a device is increased to accommodate a radiation fin. Another problem arises in that there is a cost increase attributable to an increase in the number of LEDs used.
As a solution to those problems, a backlight unit has been proposed, in which an air region having a predetermined thickness is provided between a light guide and a diffusing plate to improve mixing of emission colors. FIG. 6 schematically shows the light guide and LED light sources used in the proposed backlight unit. As shown in FIG. 6, LED light sources 115 are provided on both of side surfaces (light entrance surfaces) 112T and 112B along the longer sides of a rectangular light-emitting surface 116 of a light guide 110 in the form of a thin plate. Both of side surfaces along the shorter sides of the light guide 110 are constituted by reflective surfaces 113L and 113R to allow high utilization of light from the LED light sources 115.
The number of LEDs 77 in each color among the LED light sources 115 is determined by the setting of white balance. For reasons associated with the amount of emission from LEDs, it is preferable to combine one each B (blue) emission LED 77B and R (red) emission LED 77R with two G (green) emission LEDs 77G. By arranging the LEDs 77 in each color at equal intervals, the colors of the individual LEDs 77 are visually perceived at substantially the same distance from a light entrance surface 112 regardless of the types of the LEDs 77. Attention is paid here to color uniformity, and a minimum unit array “GBGR” is therefore employed to space two green emission LEDs 77G away from each other in a minimum unit array constituted by a B (blue) emission LED, an R (red) emission LED, a G (green) emission LED, and another G (green) emission LED and to prevent the green emission LEDs 77G from adjoining each other even when minimum unit arrays are arranged consecutively.
Therefore, the LED light sources 115 include LED sets (hereinafter also referred to as “GBGR” sets as occasion demands) 100 located at side ends of the reflective surface 113L, the LED sets being a series of LED sets starting at the ends of the reflective surface 113L and each consisting of a G (green) emission LED 77G, a B (blue) emission LED 77B, another G (green) emission LED 77G, and an R (red) emission LED 77R provided in the order listed. A plurality of the LED light sources 115 are arranged in series from the left ends of the light entrance surfaces 112T and 112B of the light guide 110, and “GBGR” sets serving as unit light sources, the width of the “GBGR” sets constituting a pitch of the light sources.
When such a configuration is employed, at an arbitrary point P2 inside the light exit surface 116, four beams of light from a G emission LED 77G, a B emission LED 77B, another G LED emission 77G, and an R emission LED 77R in the neighborhood of the arbitrary point P2 can be mixed to generate a beam of light in a desired color.
Let us now discuss an arbitrary point P1 which is located, for example, in the neighborhood of the top of the reflective surface 113L along the left shorter side of the light exit surface 116. Light arriving at this point includes not only direct light from the “GBGR” set 100 at the left end of the light entrance surface 112T but also light from an “RGBG” set 101 which is a mirror image generated as a result of reflection of the light from the “GBGR” set 100 at the reflective surface 113L. The result is equivalent to arranging a set of eight LEDs, i.e., R, G, B, G, G, B, G, and R LEDs in the order listed from the left side of the figure in the neighborhood of the arbitrary point P1. Beams of light from a four-LED set, i.e., a “BGGB” set that is located closer to the arbitrary point P1 among the eight LEDs are mixed with each other at relatively high intensity. Even if the LED set of interest is expanded to include six LEDs closer to the point, it becomes a “GBGGBG” set which includes no “R” LED. As a result, mixed white that lacks a red component is generated at the arbitrary point P1, and a problem therefore arises in that a color irregularity is liable to occur in the neighborhood of the top end of the reflective surface 113L. Such a color irregularity can occur also at the bottom end of the reflective surface 113L and the top and bottom ends of the reflective surface 113R on the right when the same LED arrangement is employed. The same problem occurs even if the LED sets 100 have a “GRGB” pattern instead of the “GBGR” pattern.
Patent Document 1: JP-A-2003-215349
Patent Document 2: JP-A-2004-95390
Non-Patent Document 1: Nikkei Electronics No. 844 pp. 126-127, Mar. 31, 2003